bpo-41376: Fix the documentation of `site.getusersitepackages()` (GH-21602) (GH-22592)

`site.getusersitepackages()` returns the location of the user-specific site-packages directory
even when the user-specific site-packages is disabled.

```
$ python -s -m site
sys.path = [
    '/home/user/conda/lib/python37.zip',
    '/home/user/conda/lib/python3.7',
    '/home/user/conda/lib/python3.7/lib-dynload',
    '/home/user/conda/lib/python3.7/site-packages',
]
USER_BASE: '/home/user/.local' (exists)
USER_SITE: '/home/user/.local/lib/python3.7/site-packages' (doesn't exist)
ENABLE_USER_SITE: False
```

It was not practical to prevent the function from returning None if user-specific site-packages are disabled, since there are other uses of the function which are relying on this behaviour (e.g. `python -m site`).
(cherry picked from commit 35f041dd0171f575fc3adce1709b31fdf45a5ff6)

Co-authored-by: Phil Elson <pelson.pub@gmail.com>
Co-authored-by: Phil Elson <pelson.pub@gmail.com>

diff --git a/Doc/library/site.rst b/Doc/library/site.rst
index b424e1ba348d870a7534d54cec06a29fcb9cec88..2e3646f6a74f80ce19d9aa7725a7388a1980a3b2 100644 (file)
--- a/Doc/library/site.rst
+++ b/Doc/library/site.rst
@@ -231,7 +231,9 @@ Module contents
 
    Return the path of the user-specific site-packages directory,
    :data:`USER_SITE`.  If it is not initialized yet, this function will also set
-   it, respecting :envvar:`PYTHONNOUSERSITE` and :data:`USER_BASE`.
+   it, respecting :data:`USER_BASE`.  To determine if the user-specific
+   site-packages was added to ``sys.path`` :data:`ENABLE_USER_SITE` should be
+   used.
 
    .. versionadded:: 3.2